1. Field of the Invention
In injection systems of the UIS (unit injector system) type, a final control element, via a hydraulic coupler, closes the high-pressure chamber of a fuel injection pump. In the UI systems, the pressure is built up by means of a piston. When a certain value is reached, the pressure opens the injection valve.
2. Description of the Prior Art
With pump-nozzle systems (UIS or unit injector systems), mechanically-hydraulically controlled injection phases are presently generated in self-igniting internal combustion engines, and these phases contribute on the one hand to noise abatement and on the other to minimizing pollution. In unit injection systems, a distinction is made among four operating states. A pump piston is moved upward via a restoring spring. The fuel, which is under constant overpressure, flows out of the low-pressure part of the fuel supply via the inlet bores, integrated with the engine block, and the inlet conduit into a valve chamber of a magnet valve. The magnet valve is opened. Via a connecting bore, the fuel enters the high-pressure chamber.
Upon a rotation of the drive cam, the pump piston moves downward. The magnet valve remains in its open position, and the fuel is pressed by the pump piston, via the return conduit, into the low-pressure part of the fuel supply.
In a third phase of the injection event, when an electromagnetic valve is used, the coil of the electromagnet is supplied with current at a certain instant by the control unit, so that the magnet valve needle is pulled into a seat, and the communication between the high-pressure chamber and the low-pressure part is closed. This instant is also known as the xe2x80x9celectrical injection onsetxe2x80x9d. The fuel pressure in the high-pressure chamber rises continuously as a result of the motion of the pump piston, and as a result a rising pressure is also established at the injection nozzle. When a nozzle opening pressure of approximately 300 bar is reached, a lifting of the nozzle needle occurs, causing fuel to be injected into the combustion chamber. This instant is also called the xe2x80x9cactual injection onsetxe2x80x9d, or as the supply onset. Because of the high pumping rate of the pump piston, the pressure continues to rise further during the entire injection event. In a concluding operating state, the coil of the electromagnet is switched off again, after which, after a slight delay time, the magnet valve opens, and the communication between the high-pressure chamber and the low-pressure part is again opened. In this transition phase, the peak pressure is reached, which depending on the pump type varies between a maximum of 1800 and a maximum of 2050 bar. After that, the pressure collapses very quickly. When it falls below the nozzle closing pressure, the injection nozzle closes and terminates the injection event. The remaining fuel pumped by the pump element until the apex point of the drive cam is forced into the low-pressure part of the fuel system via the return conduit.
As a rule, such unit injector systems, used for instance in injection systems for trucks, include two final control elements embodied as electromagnets. When there are two final control elements, the unit injector system requires more space, which when such injection systems are used in utility vehicles is available. It is therefore easily possible to accommodate the final control elements embodied as magnet valves in Diesel engines for trucks. In passenger cars, however, there is only limited space available in the engine compartment, especially in the upper region of the cylinder block, and this codetermines the outer dimensions of a unit injector system. If instead of final control elements in the form of two electromagnet valves, only one electromagnet valve is used, then there is the disadvantage that the valve can be kept in an intermediate position only highly imprecisely and at high effort and expense in terms of regulation.
The advantage of the embodiment proposed according to the invention is above all that the closure of the high-pressure chamber of the unit injector system can be controlled separately for the pressure buildup and for the opening of the nozzle needle. Thus the injection pressure, that is, the pressure at which the nozzle needle opens, can be varied freely.
If a piezoelectric actuator is used, then its stroke length can be varied arbitrarily by varying the voltage at the piezoelectric actuator. The final control element, embodied as a piezoelectric actuator, of the unit injector can be followed downstream by a hydraulic pressure booster, in order to achieve the requisite stroke lengths within the housing of the unit injector. An final control element embodied as a piezoelectric actuator can be switched to various switching positions by means of supplying suitable current. In the voltage-free state of the piezoelectric actuator without any current, the nozzle needle assumes an open position; that is, fuel is pumped at low pressure by the fuel feed pump through the unit injector. In this position of the nozzle needle, the low-pressure side inlet and the inlet to a high-pressure side pump chamber communicate with one another by a short circuit, since the openings corresponding to them inside the housing of the unit injector are uncovered by the nozzle needle.
At maximum current supply to the piezoelectric final control element, the nozzle needle is moved downward; that is, the low-pressure side inlet and the high-pressure side inlet to the pump chamber are both closed. The pressure buildup begins in this phase, without the nozzle needle being opened by the steadily rising pressure. Conversely, once the desired injection pressure has been built up in the unit injector, the current supply to the piezoelectric final control element is withdrawn, as a result of which the nozzle needle opens with pressure reinforcement.
The end of the injection is brought about by opening the return, which causes the closure of the nozzle needle. With this provision, with an final control element embodied as a piezoelectric actuator, the pressure buildup and the opening of the nozzle needle can be controlled separately from one another in a unit injector, which allows freedom of choice in terms of the injection pressure built up.